Digital image data is input to a system adapted to visually display digital image data on a display device. Digital image data is input to a frame locked system one frame at a time at an input frame or vertical refresh rate. A frame is an image displayed for viewing on a display device or panel at one time, i.e., one frame of data fits on the display device screen or panel. Each frame includes a rectangular array of pixels. Each pixel has one or more values, for example, a gray scale luminance value for a monochrome display or red, green, and blue (RGB) luminance values for a color display. The resolution of the array, i.e., the number of horizontal and vertical pixels, is often referred to as an image frame resolution. Common display frame resolutions include that shown in Table 1 indicating, in the second and third columns, the number of pixels in the vertical and horizontal dimensions, respectively:
TABLE 1VGA 640480SVGA 800600XGA 1024768SXGA12801024 UXGA16001200 HDTV1280720
Display devices must be refreshed many times per second. The frame rate for a display device is measured in hertz (Hz). Digital image data is input at an input frame rate. An input frame rate is the rate at which a frame of data is received by the system. An output frame rate is the rate at which digital image data is provided to a display device for visually displaying the input image data. Common input and output frame rates include 60, 75, and 85 Hz and the like.
Where the input frame rate and/or resolution match the output frame rate and/or resolution, the frame of image data is displayed directly without issue. If, however, the input and output frame rates and/or the resolutions differ substantially, the image data might not be properly displayed on the display device. This is particularly true in frame locked systems where small line memories are commonplace since these line memories do not allow for full conversion of the input frame rate to an output frame rate that matches the display frame rate.
The system displays an image by enabling or activating discrete picture elements (pixels) contained within the display device. The system enables each pixel by successively scanning horizontal lines of the pixel array responsive to the digital image data. That is, the system scans a line of the pixel array, retraces, scans a next line of the pixel array, and so on, activating individual pixels during each scan. The rate at which the system scans each line horizontally is termed the horizontal display frequency or pixel rate.
A source typically encodes incoming source digital image data at a source pixel rate. A display, in turn, operates at a display pixel rate. In general, it is desirable for the source pixel rate to equal the display pixel rate for a proper reproduction of the image. The industry trend is to provide digital image data to displays at higher and higher source pixel rates because they enable higher frame rates and/or resolutions that, in turn, lead to higher quality images. The display pixel rates, unfortunately, have not kept up with the increase in source pixel rates. Oftentimes it is desirable to display an image having a source pixel rate that is different, e.g., higher, than a display pixel rate.
Accordingly, a need remains for a system and method for converting a frame or pixel rate of an incoming digital image frame.